1. Field of the Invention
The present invention relates to an optical transmission system, and particularly to a dispersion compensation control apparatus in an optical transmission system employing optical phase modulation and demodulation.
2. Description of the Related Art
In recent years, there have been increasing demands for introduction of a 40 Gb/s optical transmission system of the next generation, and transmission distance and frequency usage efficiency equivalent to a 10 Gb/s system have additionally been sought in the 40 Gb/s system. As means for realizing the system, research and development of RZ-DPSK (Return to Zero-Differential Phase Shift Keying) modulation and CSRZ (Carrier Suppressed Return to Zero)-DPSK modulation, which excel in an optical signal-to-noise ratio (OSNR) tolerance and nonlinear tolerance compared with NRZ (Non Return to Zero) modulation employed in the conventional systems with 10 Gb/s or less, have been opened up. In addition to the above modulations, research and development of phase modulation method such as RZ-DQPSK or CSRZ-DQPSK (Differential Quadrature Phase Shift Keying) modulation having the characteristics of narrow spectrum (high-frequency use efficiency), has also become active. Here, the RZ-DQPSK modulation is a modulation method applying RZ intensity modulation to DQPSK-modulated light, and the CSRZ-DQPSK modulation is a modulation method, when applying RZ intensity modulation to DQPSK-modulated light, for generating an RZ signal with a carrier having narrowed spectrum range by contriving the phase.
FIGS. 1A through 1C are diagrams showing an optical transmitter apparatus for transmitting optical signals employing 43 Gb/s RZ-DPSK or CSRZ-DPSK modulation method, and an optical receiver apparatus for performing receiving processing such as demodulation of the optical signals modulated by the RZ-DPSK or CSRZ-DPSK modulation.
In a case of transmitting/receiving optical signals by the RZ-DPSK or the CSRZ-DPSK modulation method, the optical intensity is 43 GHz clock waveform, and information is carried by binary optical phase.
An optical transmitter apparatus 10 shown in FIG. 1A comprises a transmission data processing unit 11, a CW (Continuous Wave) optical source 12, a phase modulator 13, and an RZ-pulsing intensity modulator 14. The transmission data processing unit 11 comprises a function as a DPSK pre-coder for performing coding, reflecting information of difference between the current code and the 1-bit previous code, in addition to a function as a framer for framing the input data and a function as an FEC (Forward Error Correction) encoder for adding error correction codes.
The phase modulator 13 modulates continuous wave from the CW optical source 12 by coding data from the transmission data processing unit 11, and outputs an optical signal with constant optical intensity, carrying information on the binary optical phase, that is a DPSK modulated optical signal. Furthermore, the RZ-pulsing intensity modulator 14 performs RZ-pulsing of the optical signal from the phase modulator 13. In other words, as shown in FIG. 1C, when the phase-modulated optical signal is in the state shown as A2, separately from this phase modulation, an optical signal shown as A1, which is RZ intensity-modulated with the same frequency as the bit rate (43 GHz), is generated. Particularly, an optical signal, which is RZ-pulsed by using a frequency being the same as the bit rate (43 GHz) and a clock driving signal having amplitude as much as extinction voltage (Vπ), is referred to as an RZ-DPSK signal, and an optical signal, which is RZ-pulsed by using a frequency half of the bit rate (21.5 GHz) and a clock driving signal having amplitude twice as much as the extinction voltage (Vπ), is referred to as an CSRZ-DPSK signal.
In addition, an optical receiver apparatus 20 in FIG. 1B is connected to the optical transmitter apparatus 10 via a transmission path 5, and performs the received signal processing of the (CS)RZ-DPSK signal, and the apparatus comprises a delay interferometer 21, an O/E converter unit 22, a Clock Data Recovery (CSR) 23, and a received data processing unit 24.
The delay interferometer 21 comprises a Mach-Zehnder interferometer, for example, and causes interference (delay interference) between a 1-bit time delay component (23.3 ps in this case) of the (CS)RZ-DPSK signal transmitted via the transmission path 5 and a 0-rad phase-controlled component. As a result of the interference, two outputs are obtained. In other words, one of the split waveguides having the Mach-Zehnder interferometer is formed so as to be longer by the propagation length corresponding to the 1-bit time than the other split waveguide, and comprises an electrode 21a for phase control of the optical signal propagated in the other split waveguide.
The O/E converter unit 22 comprises dual pin photodiodes for performing balanced detection by receiving each of the two outputs from the above delay interferometer 21. Note that, the received signal detected in the above O/E converter unit 22 is amplified appropriately by an amplifier 22c. CDR 23 extracts a data signal and a clock signal from the received signal detected by the balanced detection in the O/E converter unit 22. Based on the data signal and the clock signal extracted in CDR 23, signal processing such as error correction is performed in the received data processing unit 24.
As other technologies relating to the present invention, there are technologies described in the following Patent Documents 1-5.
[Patent Document 1]
U.S. Patent Application Publication No. 2004-0223769
[Patent Document 2]
Japanese Patent Application Publication No. 08-321805
[Patent Document 3]
Japanese Patent Application Publication No. 2000-115077
[Patent Document 4]
Japanese Patent Application Publication No. 2003-60580
[Patent Document 5]
Japanese Patent Application Publication No. 2004-516743
The above optical receiver apparatus, however, may require highly precise dispersion compensation by placing a variable chromatic dispersion compensator (VDC) 25 in the receiver end, as shown in FIG. 1B, because in 40 Gb/s or 43 Gb/s transmission, the wavelength dispersion tolerance becomes one-sixteenth of the tolerance in the 10 Gb/s transmission.
In such a case, the optical receiver apparatus needs to have optimal setting of the amount of dispersion compensation in VDC, in addition to optimal setting of the phase control in the delay interferometer. That is, in order to receive the (CS)RZ-D(Q) PSK modulated optical signal, the optimal settings of both delay interferometer and VDC are required in order to demodulate the modulated optical signal.
In view of the above point, for dispersion compensation, it is assumed that the number of errors is monitored by error correction number etc. of the decoded received signal, and the VDC is controlled based on the monitored number of errors. However, the relation with the number of errors are different between the characteristics of the amount of dispersion compensation and the characteristics of the amount of phase control by the nature. At the initial setting, because the amount of control is not at the optimal value in both of the delay interferometer and VDC, relatively long time is required in order to find the amount of control, which is optimal for both devices and to improve the quality of the received signal, posing a problem for prompt stabilization of the amount of control of the delay interferometer and of VDC.
In other words, by the optical phase control in the above delay interferometer and the control of the amount of dispersion compensation by VDC, the above number of errors changes, and therefore, it is difficult to stabilize the amounts of control of the both devices immediately after the initial device start-up.
Additionally, since the optical phase difference may fluctuate in the transmission path wavelength dispersion and the delay interferometer due to the terminal changes during the system operation, adaptive control of the delay interferometer and VDC is required. In the technologies described in Patent Documents 1-5 and other conventional art, such a combination of controls in the delay interferometer and VDC of the phase modulation method had not been investigated.
FIG. 2 is a diagram showing a concept of obtaining the optimal value in a case of performing the adjustment of the amount of phase control and the adjustment of the amount of dispersion compensation at the same time in the conventional configuration.
As shown in FIG. 2, in the conventional configuration, adjustment of the amount of phase control after adjusting the amount of dispersion compensation is repeated so as to gradually approach to the optimal value. However, as is clear from FIG. 2, in terms of the amount of dispersion compensation, for example, a control fluctuates between the higher and lower values centering on an optimal value, and it is considered as repeating wasted motions in order to find the optimal value. By so doing, during the process of optimizing both of the amount of dispersion compensation and the amount of phase control, wasted motions are increased, preventing the prompt setting of optimal value of the amount of dispersion compensation and the amount of phase control.